Activated carbon has long been used for the adsorption of odors and other objectionable compounds. The term xe2x80x9cadsorptionxe2x80x9d generally refers to the preferential partitioning of substances from a gaseous or liquid phase onto the surface of a solid substrate. Adsorption is not the same as absorption, where a liquid being absorbed interpenetrates the absorbing phase. Physical adsorption is believed to be caused mainly by van der Waals forces and electrostatic forces between adsorbate molecules and the atoms which compose the adsorbent surface.
In spite of its excellent properties as an adsorbent, the use of activated carbon in disposable absorbent articles such as diapers or sanitary napkins has been limited by its black color. Activated carbon granules in a pouch may also make unwanted noise or provide an undesirable gritty feel when incorporated into an article worn against the body. What is needed is an improved means of adapting activated carbon for use in absorbent articles or other products wherein the color is changed from black and, optionally, wherein the physical properties of the activated carbon are modified to improve tactile properties or reduce noise or achieve other desirable improvements in the function of the activated carbon material.
It has been discovered that activated carbon particles can be treated to have arbitrary colors such as green, blue, red, or gold without a significant loss of adsorptive properties. The color of activated carbon can be changed by coating the particles with a coating material comprising a binding agent and a masking agent such as a pigment or dye. The coating material can have sufficient diffusivity or permeability to permit at least one selected odoriferous agent to be adsorbed at an efficiency relative to the uncoated activated carbon of at least 30%, more specifically at least 50%, more specifically still at least 70%, with exemplary ranges of from about 60% to about 95% or from 75% to 100%.
The binding agent can be water insoluble, allowing the coating material and the dye or pigment to remain in place even when the coated activated carbon particles have been wetted. The cured or dried binding agent and/or coating material can also be deformable, having a low degree of hardness to improve the physical properties (e.g., tactile or acoustic properties) of the coated particles when used in an absorbent article. The deformable coating can be elastomeric, comprising an elastomeric binding agent such as a silicone or latex. A silicone binding agent, for example, offers good diffusivity to some odoriferous species such as trimethylamine or ammonia, and can allow the species to be adsorbed almost as efficiently with a colored coating as without the coating. In one embodiment, a non-tacky, elastomeric coating comprising a colored pigment or dye can yield treated particles that flow relatively freely with low noise and that do not feel as rough or gritty as untreated particles when held near the skin in a pouch.
As used herein, the xe2x80x9cadd-onxe2x80x9d level of the coating material is refers to the mass of coating material relative to the mass of the uncoated activated carbon. It is calculated by dividing the mass of the applied coating material (after drying or curing is complete) by the mass of the dry, uncoated activated carbon and multiplying by 100%. The coating material can be applied at an add-on level relative to uncoated activated carbon of greater than about 5%, such as from about 5% to 300%. More specifically, the add-on level can be from about 10% to 250%, more specifically still from about 15% to 200%, more specifically still from about 20% to 100%, and most specifically from about 25% to about 80%. The coating material can comprise up to about 95% by weight masking agent, such as from 5% to 95%, 10% to 80%, 30% to 80%, and 40% to 75%.
The masking agent provides opacity and optionally color to the coating material, and can comprise a mineral such as titanium dioxide (anatase, rutile, or other forms), kaolin, silica, alumina, calcium carbonate, calcium sulfate, calcium bicarbonate, mica, barium sulfate, zinc oxide, magnesium oxide, aluminum trihydroxide, and zirconium oxide, and any known coloring agent such as colored pigments, including C.I. Pigment Green 50, C.I. Pigment Yellow 53, and C.I. Pigment Yellow 28, as well as various lakes (blue lake, red lake, yellow lake, and the like). Inorganic and organic pigments are available from many sources, such as DeltaColors, Inc. (Lawrenceville, Ga.) or BASF Corporation (Mount Olive, N.J.), which produces pigments and colorants under the Sicomet(copyright) and Sicovit(copyright) trademarks. Inorganic pigments made of minerals can be extracted from earths, fossils, marble or other volcanic and sedimentary rocks in the form of silicates, carbonates, oxides, sulfides and the salt of various metals, such as iron. The masking agent can comprise a white mineral such as titanium dioxide and a dye or colored pigment.
Median particle size of particles in the masking agent, as determined by a Coulter LS100 laser diffraction particle size analyzer manufactured by Beckman Coulter, Inc. (Fullerton, Calif.), can be about 20 microns or less, more specifically about 6 microns or less, and most specifically about 2 microns or less such as less than 1 micron. For example, TiO2 is commonly available in submicron grades having median particles sizes from about 0.25 to about 0.6 microns. Alternatively, masking agent particles can have a Hegman fineness of at least 6 NS (about 25 microns or less), at least 6.5 NS (about 20 microns or less), or at least 7.5 NS (about 6 microns or less). The Hegman gauge indicates the approximate size of the largest particles in a powder and is not directly related to the particle size distribution.
Without wishing to be bound by theory, it is believed that solid particles serving as a masking agent in the coating material can improve transport of odoriferous agents across the coating material (relative to activated carbon coated with a particle-free coating material) by forming micropores or channels for passage of gas to the activated carbon. In a possibly related manner, it is known that microparticles present in a polymeric film can increase the breathability of the film, especially if the film is strained with the particles embedded in it. Particles in the coating liquor may help break up a film as the coating liquor dries or cures, and leave open pores providing access to the surface of the activated carbon. Other adsorption and transport mechanisms may play a role, as well.
Activated carbon materials can be coated using any known suitable method, as described in more detail hereafter. Generally, coating the material comprises a contacting step, in which activated carbon particles or fibers are contacted with a coating liquor that can be a slurry, solution, or resin, and a curing, drying or heating step to remove water from an aqueous emulsion or to remove other liquids, or to permit curing or crosslinking of a resin. For binding agents that cure at room temperature, passage of time may be enough to complete this step, though care should be taken to prevent excessive agglomeration of coated particles by agitation, fluidization, or other means until curing or drying is complete. Once the particles have been coated, they can then be incorporated into any number of absorbent articles using any known process, such as filling a flexible, porous pouch with a quantity of the particles and then incorporating the pouch into a specific region of an absorbent article such as a diaper or ostomy bag.
The activated carbon particles of the present invention can be used in any known application of activated carbon, particularly those in which the activated carbon may be visible. Activated carbon can be beneficially used in absorbent articles such as diapers, incontinence briefs, sanitary napkin, ostomy bags, wound coverings, bed pads, shoe pads, helmet linings, apparel for hunters where suppressing body odor is desirable while in pursuit of game, athletic apparel, and the like. In absorbent articles such as diapers, the activated carbon may be placed in a region likely to be wetted by urine, or may be placed to the sides of such a region to maintain dryness of the activated carbon. Odor control is also a critical need in face masks worn by medical personnel, where activated carbon according to the present invention can be used. Some forms of surgery result in unpleasant odors, as occurs when human tissue is burned by laser or other devices, or when gastrointestinal procedures are necessary. Activated carbon, particularly in the form of thin, flexible fabrics, can play a useful role in eliminating such odors from the air breathed by medical personnel, and can also be used to remove harmful fumes.
The treated activated carbon, either as a fabric, a particulate, or a particulate bonded to a web or film, can be in regions that are likely to remain dry, such as waistbands or leg cuffs, or can be in the absorbent core of the article, where applicable. In sanitary napkins for feminine care, for example, the treated activated carbon material may be directly in the central target zone of the article, or may be disposed toward the longitudinal ends of the article or in lateral wings of the article which are not likely to become wetted. Flaps or wings for sanitary napkins are exemplified in the following patents: U.S. Pat. No. 4,701,178, xe2x80x9cSanitary Napkins with Flaps,xe2x80x9d issued Oct. 20, 1987 to Glaug et al.; U.S. Pat. No. 5,267,992, issued Dec. 7, 1993 to Van Tilburg; and U.S. Pat. No. 5,346,486, xe2x80x9cSanitary Napkin Having Laterally Extensible Means For Attachment To The Undergarment Of The Wearer,xe2x80x9d issued Sep. 13, 1994 to Osborn et al.